Digital satellite video broadcast has been tremendously successful throughout the world ever since the emergence of the Digital Video Broadcasting via Satellite (DVB-S) standard. However, the DVB-S is based on the state of art technologies available at the inception of the standard; these technologies are now dated. As signal processing technologies evolve—in particular, the breakthrough in error correction coding—more efficient ways to utilize the spectral and satellite powers are possible. Unfortunately, any introduction of new technologies to a successful platform likely entails service interruption and absorbing the heavy cost of replacing the legacy equipment. In this scenario, there are millions and millions of receivers already deployed based on DVB-S. The replacement of these equipment would be astounding, running into the billions of dollars. Therefore, there is a need for a mechanism to provide a smooth, cost effective transition from the current platform to the next generation systems.
It is recognized that hierarchical modulation provides such a mechanism to structure the signal constellation into two layers: one layer (the upper layer) conforming with the legacy technology (therefore equipment), and the other layer (the lower layer) supports new services. In this configuration, the upper layer signal can be received by both the legacy and new equipment. Importantly, this will not interrupt the legacy service, and the new equipment can receive both layers to enjoy the additional services. On the other hand, this approach is less spectral and/or power efficient than a non-backward compatible scheme. Also, the new receiving equipment will need to be equipped with non-backward compatible modes as well. As the legacy equipment is phased out, the system can gradually transition to the non-backward compatible modes, perhaps on a transponder by transponder basis.
In systems employing hierarchical modulation, rapid and efficient signal acquisition and frame synchronization poses a challenge. Traditionally, frame synchronization has not been an area of major concern for conventional broadcast and/or continuous transmission systems employing convolutional code since decoding can be performed prior to frame synchronization. The post decoding frame synchronization can benefit from the coding gain offered by the error correction codes. For instance, the DVB-S standard has been widely adopted worldwide to provide, for example, digital satellite television programming. Traditional DVB compliant systems employ fixed modulation and coding schemes. At present, such DVB compliant systems utilize Quadrature Phase Shift Keying (QPSK) modulation and concatenated convolutional code and Reed-Solomon channel coding. Given the fact that modulation and coding schemes are fixed, and the fact that the continuous transmission nature of broadcasting or unicasting, a simple framing structure can be utilized for these applications. In actuality, the only framing overhead is a Synchronization (“SYNC”) byte attached to a MPEG 2 (Moving Pictures Experts Group-2) frame. The SYNC byte is treated the same as other data by the convolutional code and the Reed-Solomon encoder. At the receiving end, the data corrupted by the communication media are first recovered by the convolutional code. The convolutional code can function without the knowledge of the framing structure. The output of the convolutional code is of high fidelity, typically at bit error rate below 1×10−5. With the high fidelity output, simple data matching with the SYNC byte is able to identify the starting point of the MPEG frame. Therefore, the transmitted data can be properly reassembled to deliver to the next layer. However, with block coded system, frame synchronization has been achieved before decoding. Especially when the receiver has to determine which modulation and coding is used among a vast amount of potential combinations of modulation and coding schemes. Modern error correction coding, such as low density parity check codes, operates at extremely low signal to noise ratios. This implies that such frame synchronization needs to be achieved at the same low signal to noise ratios. Furthermore, frame synchronization in such system is more than to find the beginning and ending point of a frame, it also needs to identify the modulation and coding scheme employed in the frame.
Under these conditions, the conventional approaches to frame synchronization do not operate well in that the requirements of high fidelity outputs, for example, can no longer be guaranteed.
Consequently, other approaches have been developed, but require incurring significant overhead (i.e., reduction in throughput) and receiver complexity. For example, one approach suggests using a forward error correction coding, such as a Bose Chaudhuri Hocquenghem (BCH) code, to protect the framing information within the frame structure. At the receiving end, the receiver searches for the unique word first by correlation. Once the unique word is detected, the BCH coded framing information is decoded coherently by a maximum likelihood correlation decoding. A drawback of this technique is that the unique word has to be large (i.e., high overhead). Another drawback is that true maximum likelihood decoding of the BCH code is quite complex. Furthermore schemes with these kinds of nature, i.e., using training symbols, cannot preserve the backward compatibility in a hierarchical modulation.
Therefore, there is a need for a frame synchronization mechanism that provides rapid acquisition without incurring large overhead costs, while ensuring backward compatibility with deployed technologies and services. There is also a need for a frame synchronization approach that is simple to implement. There is also a need to provide a synchronization technique that is flexible as to provide coding and modulation independence.